1. Field of the Invention
This invention relates to a method for the production of pyromellitic anhydride from a tetraalkyl benzene by the catalytic vapor-phase oxidation technique. Pyromellitic anhydride is used for heat-resistant resins, plasticizers, epoxy resin curing agents, etc. and is useful as various industrial materials.
2. Description of the Prior Art
Various methods have been heretofore proposed for the production of pyromellitic anhydride. Various methods of catalytic vapor-phase oxidation of 1,2,4,5-tetraalkyl benzenes discloses in JP-B-49-9,451 and JP-B-04-15,020, a method of liquid-phase oxidation of a 1,2,4,5-tetraalkyl benzene (JP-A-61-27,942), a method of liquid-phase oxidation of a 2,4,5-trimethyl benzaldehyde (JP-A-57-38,745, etc.), and a method of catalytic vapor-phase oxidation of anthracene (JP-A-56-8,388, etc.) are examples. Among other methods cited above, the method of catalytic vapor-phase oxidation of a 1,2,4,5-tetraalkyl benzene has been drawing attention as a process capable of inexpensively producing pyromellitic anhydride on a quantity basis because a zeolite type catalyst recently developed for use in the catalytic vapor-phase oxidation under consideration has opened up the possibility that the raw material 1,2,4,5-tetraalkyl benzene which has been heretofore expensive will be procured abundantly and inexpensively.
The catalysts proposed to date for use in the production of pyromellitic anhydride by the catalytic vapor-phase oxidation of tetraalkyl benzenes include V.sub.3 O.sub.5 --TiO.sub.2, WO.sub.2 type (Belgian Patent No. 655686), V.sub.2 O.sub.5 --P.sub.2 O.sub.5 --TiO.sub.2, MoO.sub.3, WO.sub.3 (JP-B-45-4,978), V.sub.2 O.sub.5 --TiO.sub.2 (anatase type)-MoO.sub.3, P.sub.2 O.sub.5 (JP-B-45-15,018), V.sub.2 O.sub.5 --TiO.sub.2 --Na.sub.2 O--P.sub.2 O.sub.5 type (JP-B-45-15,252), V.sub.2 O.sub.5 --MoO.sub.3 --P.sub.2 O.sub.5 (JP-B-47-30,821), V.sub.2 O.sub.5 --TiO.sub.2 --P.sub.2 O.sub.5 --Nb.sub.2 P.sub.5 --K.sub.2 O, P.sub.2 O.sub.5, TiO.sub.2, Na.sub.2 O (JP-B-49-31,973), and V.sub.2 O.sub.5 --B.sub.2 O.sub.5 (JP-B-48-35,251), V.sub.2 O.sub.5 --Na.sub.2 O--MoO.sub.3 --Cr, Mn, Nb (JP-A-01-294,679), for example.
As examples of an operation which uses two or more divided catalyst beds per production unit, U.S. Pat. No. 4,665,200, etc. disclose methods which repress the amount of reaction at the hot spot of a catalyst bed by such measures as diluting a catalyst with a carrier, enlarging the diameter of component beads of a catalyst, decreasing the amount of a catalyst deposited on a carrier, decreasing the content of vanadium, an alkali metal, or phosphorus, or decreasing the specific surface area of ZrO.sub.2, TiO.sub.2, or SnO.sub.2 for the purpose of lowering the temperature of the hot spot.
As an example of an operation which uses a plurality of species of catalyst unlike divided catalyst beds, JP-A-50-30,838 discloses a method for producing pyromellitic anhydride of high purity by a procedure which comprises first forming a gas in a reaction vessel and then passing the formed gas through an aftertreating vessel packed with a catalyst having a different composition from the catalyst used in the reaction vessel thereby disposing of a secondary product of the reaction.
The reaction for converting a tetraalkyl benzene into pyromellitic anhydride inherently entails oxidation of four alkyl groups. It is inferred, therefore, that this reaction proceeds through a reaction path more complicated and greater in number of stages than the conventional reaction as in the production of phthalic acid from orthoxylene. Further, as the precursor of pyromellitic anhydride which is an intermediate oxide, the presence of aldehydes and dimethyl phthalic acid having two of the four alkyl groups thereof already oxidized has been demonstrated. In spite of these facts, a method embodying an idea of using as many catalyst beds as reaction stages which are involved in the production of pyromellitic anhydride by the catalytic vapor-phase oxidation of a tetraalkyl benzene has never been disclosed to date.
Since the conventional methods utilize practically one species of catalyst for effecting many separate reaction stages, they entrain various problems such as unduly low concentration of tetraalkyl benzene in the composition of the raw material gas, insufficient selectivity of the conversion of tetraalkyl benzene to pyromellitic anhydride, necessitation of a refining step for a product so defiled as to assume a color or suffer from unduly low purity notwithstanding such other conditions as mentioned above are rather satisfactory. Thus, these conventional methods do not prove fully satisfactory from the industrial point of view.
Even catalysts which satisfy these conditions to a rather large extent have their performances based on a delicate balance of their characteristic properties manifested to a plurality of reactions, their performances tend to fluctuate. The catalysts themselves, therefore, are difficult to manufacture.
An object of this invention, therefore, is to provide a novel method for the production of pyromellitic anhydride.
Another object of this invention, in association with the production of pyromellitic anhydride by the catalytic vapor phase oxidation of a raw material mixed gas comprising of a tetraalkyl benzene and a molecular oxygen-containing gas, is to provide a method for the production of pyromellitic anhydride of high purity efficiently from the viewpoint of commercial production and in a high yield by using a catalyst system which combines specific catalysts.
Yet another object of this invention is to provide a method for producing pyromellitic anhydride of sparing coloration and high purity at a high raw material concentration by the catalytic vapor-phase oxidation of a tetraalkyl benzene.